The present invention relates, in general, to apparatus for producing ultrashort (femtosecond) laser output pulses in the ultraviolet wavelength range. More particularly, the invention is directed to apparatus for producing ultrashort ultraviolet light through frequency conversion of pulses generated by lasers operating in the visible or near infrared frequency ranges.
There is a need to obtain femtosecond (10.sup.-15 sec.) pulses throughout the optical spectrum. Most of the experimental results in the femtosecond time domain obtained to date have been derived from the rhodamine 6G-diethyloxadicarbocyanine iodide (Rh6G-DODCI) colliding-pulse passively mode-locked (CPM) dye laser operating at about 620 nm. Recent successes in finding other dye combinations that can produce ultrashort pulses at other wavelengths are limited to the visible and near-infrared.
Ultraviolet femtosecond (fs) pulses at 310 nm have been generated through extracavity or intracavity frequency doubling, but in the case of extracavity operation, the pulse repetition rate is drastically reduced from the repetition rate of the CPM laser. In the case of intracavity frequency doubling, potassium dihydrogen phosphate (KDP) has been used as the doubling crystal. However, the conversion efficiency is relatively low, and no measurement to establish the pulse width of the ultraviolet pulses has been reported. Measurements of high repetition rate ultraviolet pulse widths present a difficult problem, for conventional autocorrelation (AC) measurements cannot be used for femtosecond UV pulses in the 310 nm range because of the lack of suitable nonlinear crystals. Cross-correlation (CC) measurement through sum frequency mixing with the fundamental femtosecond pulse to 210 nm is a possible means of determining the UV pulse width, but there is a dearth of good nonlinear optical crystals in this short-wavelength region. Beta-barium borate .beta.-B.sub.a B.sub.2 O.sub.4, or BBO) is the only nonlinear crystal that has a large enough nonlinear optical coefficient for phase-matched sum-frequency mixing in this spectral range, and it is transparent down to 190 nm. BBO has, however, a relatively large group velocity dispersion which tends to broaden the generated femtosecond pulses and this makes the pulse width measurements difficult unless extremely thin crystals are used. Such crystals, in turn, are difficult to fabricate and polish.